Multiplexing commutators



y 6, 1953 w. A. TOLSON 2,833,862

MULTIPLEXING COMMUTATORS Filed Sept. 11, 1953 VOLTAGE 0N GRID 0F ruse 1 SOURCE VOLTAGES -i -o 0'6 Q e I" JNVENTOR. WILLBAM A. TOLSON ATTYS.

United States Patent 2,833,862 MULTIPLEXING COMMUTATORS William A. Tolson, Hightstown, N. J., assignor, by mesne asslgnments, to the United States of America as represented by the Secretary of the Navy Application September 11, 1953, Serial No. 379,747 Claims. (Cl. 179--15) This invention relates to multiplex systems and is particularly directed to means for sequentially transmitting on a single circuit a plurality of separate distinct signals.

Time sharing is one of the more practical methods of transmitting a plurality of signals on a single circuit or channel. In high speed commutation, unfortunately, the residual voltage eifects of one signal is often carried into the next succeeding signal, and cross talk results. Where the signals to be transmitted are direct current voltages the absolute or relative values of the several signals must be carefully preserved or else intelligence may be completely lost.

7 In certain radar applications for example, it becomes necessary to store information as to the amplitudes of a multiplicity of signals and it becomes desirable to transmit this information over a signal channel by the process of time sequence sampling. It is in such applications that the relative amplitudes of the various signals must be maintained with a high degree of accuracy, and where the signals are direct current voltages it is particularly difi'icutl to prevent charges stored in the capacities of the system by one signal from eifecting the direct current voltage of the next subsequent signal.

An object of this invention is to provide an improved multiplex system.

A more specific object of this invention is a commutator system for sequentially transmitting a plurality of signals over a single circuit or channel.

A still more specific object of this invention is a cornmutator system for multiplexing a plurality of signals on a single circuit and for eliminating interference and cross talk between signals.

Other objects of this invention will become apparent in the following description of one physical embodiment thereof. The scope of this invention is defined with particularity in the appended claims and said embodiment is illustrated in the accompanying drawing in which:

Fig. 1 shows a typical wave form of voltages of a single circuit carrying the multiplexed signals of this invention,

Fig. 2shows the voltage and time relations of the several signals to be transmitted, and

Fig. 3 is a circuit diagram of the improved commutator system of this invention.

The several signals to be transmitted over a single circuit may comprise relatively steady voltages, the only distinction of theseveralsignals being the relative voltage amplitudes of the signals. Tirnewise, the signals may be considered direct current as suggested in Fig. 2 although the amplitudes may vary slowly or rapidly. That is, the commutating speed is high and the commutating cycle is short with respect to the duration of the signal increments to be transmitted. Since the shortest period of signal variation must be greater than the time sharing period required by the commutator system, the various signal voltages at, b, c, a, et cetera may be represented as direct current voltages as in Fig. 2, with the time base appropriately scaled. Each signal is successively sam- ICC pled, according to this invention, and transmitted on a single circuit in some such a wave form as that of Fig. l, where the voltage platforms a, b, c, d, et cetera represented by their heights the corresponding signal voltages a, b, c, d.

Since the absolute or relative voltage value is the only distinguishing characteristic of each transmitted signal the voltage of any one signal must not be permitted to influence or affect the voltage of any other signal. According to an important and characteristic feature of this invention the transmitted voltage is forcibly returned to a fixed predetermined level between each signal transmission. This predetermined or reference level may be high or low with respect to the signal voltages and, conveniently, may be ground potential. By reducing the voltage of the circuit carrying the signals to the reference or ground level after each signal transmission, residual voltages cannot be carried from one signal into a subsequent signal, and cross talk is effectively eliminated.

The commutator shown in Fig. 3 comprises a shaft 1 driven by the motor 2. On the shaft is mounted an insulted arm 3 which terminates in a small metal surface or tip 4 serving as one plate of a capacitor. Mounted concentrically with shaft 1 is a ring structure comprising, a plurality of segments or capacitor elements 5. Brush 6 makes electrical contact with the rotating arm 3 through a slip ring surface 7 on the hub of the arm. The hub and the arm are electrically separated from the shaft by the insulating sleeve 8. Signal sources a, b, c, d, et cetera to be coupled with the slip ring circuit, are connected, respectively, to alternate segments 5 of the commutator, the alternate segments 5g being connected together and to a common steady potential source or to ground as shown. The transmission circuit further comprises an amplifier it) with an output circuit 11 coupled, for minimum amplifier distortion across the cathode resistor 12. Thus the capacity between the commutator segments 5, 5g and tip 4 of the rotating arm is in series between the signal source and the rid of the amplifier tube. The capacities between the various signal circuit elements. and ground may be represented by the condenser 13. In operation, the commutator arm moves, alternately past a live segment and a grounded segment of the commutator.

To insure that the amplifier grid returns to a reference or ground potential after each signal transmission, the grid is coupled to ground through a clamp circuit, the impedance of which is reduced to near zero after each transmission. This intermittant change in impedance may be effected, for example, by a double diode clamp of the type described in the patent to Wendt, 2,299,945, issued Gctober 27, 1952. The grid of the signal amplifier is connected to the anode 14 of one diode and to the cathode 15 of another diode, the remaining diode electrodes, anode 16 and cathode 17, being connected through the load resistors 13 and 19 and through the potentiometer 2 Wiper 21, and resistor 21a to ground. Resistors 18 and 19 should have like and relatively high hrnic values.

According to this invention the diode electrodes 16 and l, are pulsed to cause the diodes to become conductive and to cause the grid to assume the potential of the central of the potentiometer. Since the grid is isolated from the ground and since the clamp circuit is balanced with respect to ground, the mentioned pulses applied to the diodes will not impose any potential on the amplifier grid.

After each signal transmission, cathode 16 of one diode is pulsed negatively and the anode 17 of the other diode is pulsed positively. The simultaneous positive and negative pulses for the clamping diodes are conveniently obtained from the amplifier 22 having both an anode load resistor 23 and a cathode load resistor 24. An increase in space current through amplifier 22, under the control of grid 25, causes at the terminal pins of the amplifier 22' a reduction in the anode potential and an increase in the cathode potential. The cathode 16 of the clamping diodes is coupled through coupling condenser 26 to the anode end of load resistor 23 and anode 17 of the clamping diode is coupled through coupling condenser 27 to the cathode end of load resistor 24. Hence, a positive pulse on grid 25 of the amplifier will simultaneously produce a negative pulse on cathode 16 and a positive pulse on anode 17. The two diodes thereupon become conductive, the voltage drop across each becomes low, and the anode-cathode junction (and the grid of amplifier Iii) assumes the potential of the midpoint of the potentiometer 20. Wiper 21 will usually be set at the midpoint unless a bias other than ground is desired on the grid. 1

Pulses are applied to the control grid 25 of the amplifier 22 from a simple timing circuit synchronized with the commutator. Such a timing circuit may comprise, for example, a tone wheel 39 having iron teeth revolving past the tips of a horseshoe magnet 31. Voltages induced in the winding 32 on the magnet are ample to positively pulse the control grid 25 for the purposes desired. The winding is energized as with the battery 33 in series. with coupling resistor 34. Coupling condenser 35 and resistor 36 apply the pulse voltages obtained from across 34 to the grid 25 in the usual manner. With the timing device mounted on or geared to the shaft carrying the commutator arm, it is a simple matter to adjust the space phase of the timer teeth with respect to the commutator arm to produce the clamp-actuating pulse voltages at the instant the commutator arm leaves each signal segment.

In operation, assume that the signal source a, b, c, a, et cetera are connected to their respective commutator signals and that the commutator arm is driven at a commercial motor speed. Assume that the metal surface t at the tip of the arm 3 is centered opposite segment 5 connected to signal source 0. If the width of the tip 4 is relatively narrow compared to the width of the segment 5 so that border or fringing efiects are minimized, the charge on the arm 3, the brush 6 and the control grid circuit assumes a value which can be represented by the voltage c of Fig. 1. This charge may be considered the charge of the condenser 13 which represents all of the capacities to ground of the control grid circuit. It is to be remembered the control grid circuit has not direct connection to ground. As the tip 4 moves on to the grounded commutator segment 5g, condenser 13 starts to discharge, but in the absence of a finite resistance path across capacity 13 the grid circuit would probably not be returned to ground or to a reference level before tip 4 moves on to the next succeeding segment 5 connected to signal source d. According to my invention, a pulse is generated across coupling resistor 34 by the tone wheel 30 at the instant tip 4 leaves the face of segment 5. This pulse, as explained, produces a negative pulse at the anode of amplifier 22 and at the cathode 16 of the clamping diode while simultaneously producing a positive pulse at the cathode of amplifier 22 and at the anode 17 of the clamping diode. If the charge of condenser 13 left by the last commutator segment 5 is positive, the charge immediately drains off through the anode-cathode space 14-16 of the clamping diode. Conversely, if the charge on condenser 13 was negative the charge would be dissipated through the cathode-anode space 15-47 of the diode. Because the bridge, comprising the resistance 18 and 19, the diode paths, and the resistances of the two halves of potentiometer 29 are balanced, the grid terminal of amplifier 16 is forced to assume the potential of the midpoint of the potentiometer 20. Accordingly, in the example assumed, the potential c, Fig. 11 moves directly to the reference or ground potential as suggested in Fig. l and the slow exponential decay of the charge on condenser 13 is avoided. If segments 5g of the commutator are of the same width as segments 5 the duration of the ground potential, Fig. 1, may be approximately the same as the duration of the signal potential thus assuring a wide separation in time of the succeeding signal voltages on the signal grid circuit.

In one multiplexing system found to effectively multiplex a large number of signals without cross talk comprising an amplifier lti of the type commercially known as 6L6, the cathode load resistor was 450 ohms, and the 6L6 plate voltage was 300 volts. The two clamp diodes were the two halves, respectively, of the tube commercially known as the 61-16, and the bridge resistors 18 and 19 were each 1.5 megohms, and the potentiometer total resistance was 1.0 megohm. The grounding resistor 2.1a was .5 megohm. The pulse driving tube 22 was a commercial pentode 6AG7 with a plate load resistor 23 of 1000 ohms and a total cathode load resistance of 1010 ohms.

If it is desired to demodulate the multiplexed signals it becomes necessary to have a synchronizing signal to properly identify the beginning of the sequence of the signal sampling. This can be accomplished in a very simple manner by connecting a normally unused commutator segment with a source of distinctive voltage such as a voltage which is higher than the maximum voltage to which any other active signal rises. The pulse so produced may be separated from the composite signal by means of amplitude separation in any one of a number of well known devices in the television art.

The system of this invention to return a single transmission circuit carrying a plurality of multiplexed signals to a predetermined fixed level after each signal transmission effectively eliminates cross talk, is simple in construction, is easy to operate, and is reliable in operation.

Many modifications may be made in the invention here disclosed without departing from the spirit thereof. The disclosed embodiment is specifically defined in the appended claims. t

What is claimed is:

1. In combination; a commutator with a plurality of circularly arranged segments; a rotary arm rotatable successively into capacity coupling with said segments, alternate segments being connected to a fixed potential, the remaining segments being connected, respectively, to a plurality of independent distinctive amplitude signal sources; a grid controlled amplifier, the grid of said amplifier being connected through a slip ring to said rotary arm; and means for returning said grid to a predetermined potential level each time said arm traverses said alternate segments including a source of voltage pulses synchronized with said rotary arm, and a clamping circuit responsive to voltage pulses for periodically coupling said grid to a point of said potential level.

2. In combination in a system for sequentially applying a plurality of separate direct current voltages on a single circuit; a commutator with a plurality of circularly arranged spaced segments and a rotary arm to successively capacitively couple to said segments; a plurality of sources of signals of distinctive amplitude connected, respectively, to said segments; a grid controlled amplifier with a control grid connected to said rotary arm; and means to return said grid to a fixed predeterminedpotential after each signal is applied to said grid, said means comprising a grid-controlled clamp circuit connected between said grid and ground and a pulse source connected to and synchronized with the rotary arm and coupled to the grid of said clamp circuit.

3. A signal multiplexing system comprising a'plurality of sources of separate signals of varied amplitude; a transmission circuit; commutator means successively coupling said transmission circuit to said sources; a grid controlled amplifier in said transmission circuit; means to return the grid of said amplifier to a reference level after the transmission of each signal, said means comprising a diode means connected between said grid and a point of reference potential; and a pulse generator synchronized with said commutator means, the pulse generator being so coupled to said diode means as to make the diode means conductive after transmission of each signal.

4. In combination; a commutator having a plurality of circularly arranged segments; a plurality of independent signal sources of various voltage amplitudes, the signal sources being connected to alternate segments, the remaining alternate segments being connected together and to a common reference potential; a rotatable member driven to successively capacitively engage said segments; an amplifier with a control grid connected to said rotatable member; a pulse generator responsive to the rotation of said rotatable member; and means to intermittently couple said control grid to a point of reference potential, the last named means being coupled and responsive to said pulse generator.

5. In combination, a multiplex signal circuit comprising; a plurality of independent sources of signals of varied amplitude; means to successively, capacitively sample each signal and apply each signal sample to the signal circuit; means to successively connect said circuit to a point of reference potential alternately with said successive sampling each signal, said last mentioned means comprising a first and-a second diode with the anode of one diode and the cathode of the other diode connected together and to said signal circuit, the remaining cathode and anode of said diodes being connected respectively through resistors to said point; a source of positive pulses connected to said remaining anode; a source of negative pulses connected to said remainin cathode; and means to produce the positive and negative pulses in time phase and in synchronism with the multiplexed signals of said signal circuit.

References Cited in the tile of this patent UNITED STATES PATENTS 20 2,299,945 Wendt Oct. 27, 1942 2,546,316 Peterson Mar. 27, 1951 2,679,551 Newby May 24, 1954 U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,833,862 William A Tolson May 6, 1958 It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Let oers' Patent should read as corrected below.

Column 2, line 32, after "the" insert "remaining",- column 3, line? 37 38, for "signals 5" read --segments 5"; line 49, for "has not" read ---has' Signed and sealed this 1st day of July 1958,

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Officer Conmissioner of Patents 

